1. Field of the invention
The present invention relates to systems and methods of controlling three axis stabilized spacecraft, and in particular to a method and system for performing solar wing thermal shock compensation using a solar wing position actuator.
2. Description of the Related Art
Thermal shock disturbance is a common problem experienced by earth orbit spacecraft. When such spacecraft enter and exit earth shadow, abrupt temperature changes cause rapid deformation of spacecraft solar wing panels, which results in significant induced spacecraft attitude disturbances.
There are a number of methods that have been employed to solve this problem, many of which are outlined in "An Evaluation of Thermally-Induced Structural Disturbances of Spacecraft Solar Arrays" by J. D. Johnston and E. A. Thornton, August 1996, which is hereby incorporated by reference herein. These conventional solutions to the thermal shock disturbance problem generally fall into two categories.
The first category of conventional solutions relates to the mechanical design of the solar wing panels. Here, critical structures are designed to minimize temperature gradients and the thermal deformation and resulting induced attitude disturbances. Such designs are disclosed in U.S. Pat. No. 5,720,453, entitled "Solar Panel Parallel Mounting Configuration," issued Feb. 24, 1998 to Mutschler et al, U.S. Pat. No. 5,620,529, entitled "Low Disturbance Solar Array," issued Apr. 15, 1997 to Bassily et al., which references are hereby incorporated by reference herein. One significant problem with this category of solutions is that they can significantly increase the cost of the spacecraft.
The second solution relies instead on the spacecraft attitude control system to compensate for the induced solar disturbances. These systems use control actuators to actively counteract disturbance torques resulting from thermal deformation of solar wing panels. Typically, this is accomplished by using traditional control actuators such as reaction wheels to compensate thermal shock disturbance.
However, controlling the spacecraft eclipse thermal transient becomes a significant challenge because of the high magnitude of solar wing thermal shock disturbance. Traditional control actuators such as reaction wheels are limited by their control torque capabilities, and are ineffective in the presence of such high magnitude disturbance. Thrusters can provide high control torque, but it costs propellant, requires complicated procedure to transit from wheel control to thruster control and back to wheel control, and changes spacecraft momentum state. Developing a dedicated actuator of high torque capability only for thermal shock is undoubtedly very costly.
An example of such a control system is disclosed in U.S. Pat. No. 5,211,360, entitled "Spacecraft Thermal Disturbance Control System, issued May 18, 1993 to Darrell F. Zimbleman, which is hereby incorporated by reference herein. This thermal disturbance control system comprises a network of distributed temperature sensors located on solar wing surfaces and a reaction wheel assembly mounted on a solar wing yoke. This is a relatively costly scheme because a dedicated control system including control electronics and microprocessors (in addition to the distributed sensor network and reaction wheel assembly) is needed to implement this scheme.
Another example of spacecraft attitude control system for compensating for thermal shock disturbance is disclosed in U.S. Pat. No. 5,517,418, entitled "Spacecraft Disturbance Compensation Using Feedforward Control," issued May 14, 1996 to Green et al., which is hereby incorporated by reference herein. During the thermal transient, this scheme feeds a predicted thermal control torque profile to the attitude control actuator to counteract thermal disturbance.
A third thermal disturbance compensation scheme is disclosed in U.S. Pat. No. 5,563,794, entitled "Repetitive Control of Thermal Shock Disturbance," issued Oct. 8, 1996 to Cosner et al., which is incorporated by reference herein. This reference discloses a learning procedure that allows the spacecraft attitude control system to learn control errors due to thermal disturbance over several eclipse thermal shock cycles. Using the information thus obtained, the control system maintains precise pointing in the presence of thermal shock disturbances.
One limitation of the foregoing techniques for minimizing thermal shock disturbances is that they are typically expensive and/or ineffectual to compensate for large disturbances.